1. Field of the Invention
The present invention relates to a wire-cut electric discharge machine which has a function of detecting upper and lower surfaces of a workpiece when machining the workpiece.
2. Description of the Related Art
A wire-cut electric discharge machine applies a voltage between poles of a wire electrode and a workpiece to generate electric discharge and at the same time, changes relative positions of the wire electrode to the workpiece, thus machining the workpiece in a desired form.
Concerning a machining result of a workpiece, dimensional accuracy, verticality, and angle accuracy are commonly required. In order to obtain a desired machining result, it is necessary to accurately obtain relative positions of a wire electrode to a workpiece. These relative positions are accurately obtained on the basis of a relationship of relative vertical positions of the wire electrode to the workpiece. In order to solve the issue of securing the relationship of relative vertical positions, Japanese Patent Application Laid-Open No. 2003-71636 discloses such technique that a wire electrode is brought into contact with an adjusting jig having a contact piece which is mounted on a workpiece mounting table, so as to adjust a vertical position of the wire electrode.
In the technique disclosed in Japanese Patent Application Laid-Open No. 2003-71636 mentioned above, it is necessary to set a machining condition after the vertical position of the wire electrode is adjusted. The machining condition commonly varies depending on a type and a diameter of a wire electrode and a material and a thickness of a workpiece, and a machining condition appropriate for machining a certain workpiece is stored in a numerical controller.
Associated with storing the machining condition in the numerical controller, a gap interval between a nozzle for guiding machining fluid to a discharging part and a workpiece needs to be set. In prior art technique, adjustment of a gap between an end of a nozzle and a workpiece has been manually performed.
In order to manually set a size of a gap between the end of the nozzle and the workpiece, such complicated operation is performed that an operator performs adjustment little by little by lowering a Z axis (vertical axis) in a running direction of a wire electrode little by little so that the gap has a designated fine value, while checking the gap with equipment such as a thickness gauge.
A machining condition of a wire-cut electric discharge machine varies depending on the thickness of a workpiece, so that an operator usually recognizes the thickness of the workpiece. Therefore, in a case where a workpiece is directly mounted on a workpiece placing table, it can be considered that a nozzle level can be automatically adjusted to correspond to the thickness of the workpiece.
Japanese Patent Application Laid-Open No. 7-299660 discloses such technique that a contact piece which is provided to an end of a nozzle is separated from a workpiece by a predetermined distance from a detected state where the contact piece and the workpiece contact with each other, so as to keep a distance between the nozzle and the workpiece at the predetermined distance. Further, Japanese Patent Application Laid-Open No. 2005-111608 discloses such technique that an end of a wire electrode is directly brought into contact with a workpiece so as to detect an upper surface position of the workpiece.
In the wire-cut electric discharge machine, a workpiece is machined by electric discharge and at the same time, fine particles of the workpiece generated by the electric discharge are dispersed in machining fluid. If these fine particles float around the wire electrode, electric discharge is generated between the wire electrode and the fine particles. Accordingly, electric discharge between the wire electrode and the workpiece, which is required for machining a shape, is reduced, generating such problem that machining speed is reduced.
In order to prevent the reduction of the machining speed due to such fine particles of the workpiece, it is necessary to remove fine particles around the wire electrode on a machining path so as to efficiently discharge to the workpiece, in the wire-cut electric discharge machine.
Commonly, a diameter of a wire electrode is very small, for example, approximately a few hundred μm, and a width of a machining groove which is formed by electric discharge is approximately a little larger than the diameter of the wire electrode. In order to effectively remove fine particles from such thin machining groove, it is necessary to increase pressure of sprayed water, so that a gap between the nozzle end and the workpiece needs to be decreased.
Even if an operator who performs machining of a workpiece with the wire-cut electric discharge machine recognizes the thickness of a workpiece, the thickness of the workpiece which is commonly used usually includes a dimensional error of approximately 100 μm to 200 μm. On the other hand, concerning adjustment of a gap between a nozzle end and a workpiece, it is sometimes required that the gap should be adjusted to be 100 μm to 200 μm depending on a machining condition. Therefore, it is impractical to automatically adjust a nozzle gap only depending on information of the thickness of a workpiece.
In the technique disclosed in Japanese Patent Application Laid-Open No. 7-299660, in order to detect contact of the nozzle end and the workpiece, a driving device for vertically driving a sensor and a nozzle and a control device which controls contact and isolation of the nozzle end and the workpiece are required to be provided. As a result, there are such problems that the wire-cut electric discharge machine becomes complicated and its cost is increased.
Further, in the technique disclosed in Japanese Patent Application Laid-Open No. 2005-111608, the wire electrode needs to be melt-cut or to be cut by an edged tool such as a cutter. In a case where the wire electrode is melt-cut, the diameter of an end of the wire electrode becomes thinner and sharper than a diameter of other part. Accordingly, when this end is brought into contact with an upper surface of the workpiece, the end part of the wire electrode is easily deformed. Therefore, it is difficult to accurately detect a position of the upper surface of the workpiece by using the technique disclosed in Japanese Patent Application Laid-Open No. 2005-111608. Further, in a case where the wire electrode is cut by an edged tool, the end part of the wire electrode is bent by the edged tool. In addition, in the case where the wire electrode is melt-cut, straightness of the wire electrode is improved by heating the wire electrode. However, in the case where the wire electrode is cut by an edged tool, the straightness of the wire electrode is not improved. Consequently, there is a problem that the position of the upper surface of the workpiece cannot be accurately detected in the same way as the case where the wire electrode is melt-cut.